Ventilation for 2 bathrooms/1 roof penetration?

I'm installing a bathroom directly adjacent to the existing one. The current vent exhausts air directly into my attic insulation. I'd like to remedy that, as well as install a proper vent in the new bathroom. I'd really like to only need 1 roof penetration. It looks like the way to do this is to have a central fan that exhausts both bathrooms. I would prefer to have a separate fan in each bathroom, and didn't know if there was some type of circular pipe with 2 separate chambers. So that I could have each bathroom have its own piping, but only 1 pipe going through the roof, if that makes any sense. Any other good solutions?

Commercial buildings do this all the time, having one big exhaust fan sucking air out of several bathrooms. The problem would be the extreme cost of this project and the fact that it would be a pain in the ass. If you have an exhaust fan in your attic, it will surely go bad after a few years and you'll be up there changing it out, plus the wiring to it would be a pain. If you put a big exhaust fan on your roof, even more expense and pain the the butt. Do yourself a favor and just put an exhaust fan in each bathroom and vent them separately through their own penetrations, be it through the roof or the soffit. You'll be better off.
Just don't do like you are thinking and connect the ducts. No matter how good you think your backdraft damper is, I can guarantee that you'll smell whatever is going on in the other bathroom.

Commercial buildings do this all the time, having one big exhaust fan sucking air out of several bathrooms. The problem would be the extreme cost of this project and the fact that it would be a pain in the ass. If you have an exhaust fan in your attic, it will surely go bad after a few years and you'll be up there changing it out, plus the wiring to it would be a pain. If you put a big exhaust fan on your roof, even more expense and pain the the butt. Do yourself a favor and just put an exhaust fan in each bathroom and vent them separately through their own penetrations, be it through the roof or the soffit. You'll be better off.
Just don't do like you are thinking and connect the ducts. No matter how good you think your backdraft damper is, I can guarantee that you'll smell whatever is going on in the other bathroom.

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I wasn't planning on connecting the ducts; what I was hoping for was a circular piece that penetrated the roof. Within that piece would be 2 separate chambers, each of which would be exhausting 1 bath. That way there are 2 separate circuits, but only one roof penetration.

what is often done is to install a rectangular "roof vent" above the rooms. Then the circular pipes are teminated under/inside it. The "backflow" preventers on exhaust fans are very rudimentary, and do not seal tightly. One fan with many inlets is an entirely differnt thing than many fans into one duct.

what is often done is to install a rectangular "roof vent" above the rooms. Then the circular pipes are teminated under/inside it. The "backflow" preventers on exhaust fans are very rudimentary, and do not seal tightly. One fan with many inlets is an entirely differnt thing than many fans into one duct.

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Right. What you're talking about is a fantech or similar setup with a central fan, correct? As another route, is it acceptable to terminate the run by going through the soffit? I know you can't just leave it lying next to it, but if I actually place a vent cap through the soffit? I'd much rather do that...

That is the "one fan with many inlets" design. My initial suggestion was to install a "common" ventilation roof vent and then run your individual fan vents up to it and terminate them inside it without any real "connection" between them or the vent.

Oh, I see. That was I guess my original question. I was looking for a special made pipe/vent to do that. So I could just buy an oversized roof vent and stuff both vent runs into it...
What about the soffits?

I would not run that hot/moist air to the soffits...first, it is pointing down, and the hot air wants to go up, carrying the moisture with it back into the attic and around the soffit area, and second, many soffits are not wide enough to run a decent sized vent to. Your best bet is out a gable endwall or through the roof. My bathroom in the condo does not have any windows, so to get some light in there as well as replace the cheapo vent fan that was there venting into the attic (like yours), I installed a tubular skylight with a vent fan kit from www.solatube.com . They've updated the design since mine was installed (mine sticks down into the room, the new one is essentially flush). During the day, the light from overhead can be the equivalent of a 300W lightbulb! It even lights the room on a moon-lit night. One of the best things I did during my remodeling. But, this now would require yet another roof penetration. Barring the SolaTube, I'd probably use one of the Fantec units and put it on a 3-way switch, maybe with a timer. It would ventilate both baths whenever on, but would be the cleanest solution.